I'm having a blast with Outwitters. It's hex-based, and asynchronous multiplayer only, but it's scratching the same itch that Advance Wars does for me. The skill-based matchmaking, while not perfect, is really pretty awesome.

Currently there's 3 teams to choose from, one of which you can play for free, and a 4th one is due within the month.

Oh, and it's made by the dev team duo that made the awesome Tilt to Live.